Some of the world's greatest architectural wonders don't feature straight vertical lines or cool curving silhouettes — there are leaning towers all over the world. Some of them were built that way, and some have just started to lean over time. Here are some of the most eye-popping photos.

The Church of Our Dear Lady at the Mountain (or Upper Church / Oberkirche), Bad Frankenhausen, Germany

The wonderful church was built in 1382, but in the 17th century its tower started to tilt, due to the growing sinkholes of nearby salt mines. Pictured above.

Torre Delle Milizie, Rome, Italy

The original height of the tower is uncertain, but after a big earthquake in 1348, the top two floors were demolished, and now it's 160 ft (50 m) high. This earthquake caused the slight tilting of the structure.

The leaning towers of Venice, Italy

Chiesa di San Giorgio dei Greci

Greek Orthodox worship was not permitted in Venice for centuries, but from the 16th Century the local Greek community gained the rights to build a confraternity and a church with a belltower. The tower was completed in 1592 — and now it leans to the west.

Chiesa di San Martino, Burano

Basilica di San Pietro di Castello

This church was estabilished by bishop Saint Magnus in the 7th Century, but the present building was built at the end of the 16th and in the first three decades of the 17th century. It contains the Throne of St Peter, a 13th century seat cut from a funeral stone and inscribed with words from the Koran.

The Leaning Tower of Suurhusen, Germany

The 89.8 ft (27.37 m) high tower was made of brick around 1450, and currently leans at an angle of 5.1939 degrees. The reason for the tilt? The whole tower is standing on oak tree trunks which were preserved by groundwater, but in the 19th century the land was drained and it made the wood rot. This is the most slanted tower in the entire world.

Söyembikä Tower, Kazan, Republic of Tatarstan, Russia

The architectural symbol of the Tatar capital was probably built in the first half of the 16th century, but several scholars date its construction to the late 17th or early 18th centuries. Hundred years ago its inclination was estimated at 76 in (194 cm). Some stabilization methods were used in the 1930s and 1990s, but the leaning is still visible.

St. Mary and All Saints, Chesterfield, Derbyshire, United Kingdom

The church has a twisted spire, added to the church in 1362. It is both twisted and leaning, twisting 45 degrees and leaning 9ft 6 in (2.9 m) from its true centre. This interesting shape was caused by the heating of the lead that covers the spire.

Greyfriars Tower, King's Lynn, West Norfolk, United Kingdom

The only remaining part of the Franciscan monastery, the 92 ft (28 m) high tower was a useful seamark for centuries. As its worst, the lean was 2 ft 2 in (67.5 cm) — which is just over 1 degree — but seven years ago, the slant was corrected. (via Geograph/Richard Humphrey)

Leaning Temple of Huma, India

The only leaning temple in the world is dedicated to the Hindu god Shiva. The main structure was built in the late 17th century, but some others were added in the 1770s and 1780s.

The Clock Tower of Teluk Intan, Malaysia

It was originally a water tower built in the late 19th century, but later it was used as a watchtower during the Japanese occupation of the country in 1941. It looks like an 8 storey building, but on the inside it is actually divided into three stories.

Qianwei Town Leaning Tower, Qianwei, China

Tower of Nevyansk, Sverdlovsk Oblast, Russia

The 18th Century tower's purpose isn't really clear: the historians are not sure that it was a watchtower, a belltower, a place to create counterfeit money, or just a symbolic tower to embody the might of the funder Demidov family. The height of the building is 189 ft (57.5 m) and it's deviation is 7 ft 3 in (2.2 m).

That's not the only mystery in the tower's history: there is a square acoustic room where if a person stands in one corner, he or she can whisper anything to another person in the opposite corner. The tower's tented room was the first cast iron cupola ín the world, hundred years before the reconstruction of the Mainz Cathedral in Germany and the workers used reinforced concrete, 130 years before a Parisian gardener discovered it in the 1860s.

Tower Of Pisa, Pisa, Italy

And finally, the most famous example of a leaning tower. This campanile (freestanding bell tower) leaned at an angle of 5.5 degrees, but after the restoration work between 1990 and 2001, now it leans at about 3.99 degrees. It means that the top of the tower is displaced horizontally almost 3.9 meters (12 ft 10 in) from where it would be if the structure were perfectly vertical. The construction of the 183.3 ft (55.86 m) tower took almost 200 years (1173-1372).

The tower has been leaning since shortly after its initial construction in 1173, due to unstable substrate soils.